1. Field of the Invention
The present invention relates to a method of driving a ferroelectric liquid crystal display element, more particularly, to a method of driving a surface-stabilized ferroelectric liquid crystal display element to increase the number of gray scales (gradations).
2. Description of the Related Art
In recent years, as office automation has advanced, use of so called OA-equipment such as word processors and personal computers has become widely spread. In particular, light and compact OA-equipment such as lap-top and palm-top devices are demanded as personal-use equipment. For this compact OA-equipment, compact keyboards and displays are needed as human interfaces. In particular, displays serving as faces of the equipment are needed not only to be light and compact but also to be flat, thin, and high quality.
Namely, in recent years, to meet the requirements of lightness, compactness, flatness, thinness, and high quality, liquid crystal displays (LCDs) are widely used. Note, the LCDs are compact, light, and thin, to consume small electric power, provide relatively high information content, and be able to display colors. Therefore, LCDs nearly satisfy the requirements for the displays of the OA-equipment.
Incidentally, a conventional supertwisted LCD (STN-LCD) may have an information content of about 1200.times.800 pixels at the maximum. Since this display has a long response time, a cursor on a screen of the display moved by a mouse cannot follow the movements of the mouse, so that it is not satisfactory as a display for a computer that uses a mouse. The STN-LCD has another problem of deteriorating a contrast ratio in proportion to an increase in the display capacity. In particular, a high resolution display with 1200.times.800 pixels achieves an insufficient contrast ratio of about only 8:1. The most serious problem of the STN-LCD is a narrow viewing angle (narrow angle of visibility), which is about only 30 degrees with respect to a normal angle to the screen. Accordingly, the contrast ratio and colors change depending on an angle of view, and therefore, the STN-LCD is not convenient for a user to use. The STN-LCDs must solve these problems.
To solve these problems of the STN-LCDs, a ferroelectric liquid crystal display (FLCD) having fast-switching and bistable surface stabilized liquid crystal (SSFLC) structure has been proposed (for example, Appl. Phys. Lett. Vol. 36, p. 899 (1980) by N. A. Clark et al). The FLCD (SSFLC device) is bistable in terms of electro-optical characteristics, so that it may materialize a high information content with use of a memory effect of liquid crystals. Since a drive time per scan line of the FLCD is very short about 100 .mu.sec., a cursor on a screen of the FLCD sufficiently follows the movements of a mouse. Liquid crystal molecules of the FLCD are always in parallel with a substrate (a glass supported substrate) irrespective of the presence of an applied electric field, so that the FLCD provides a very wide viewing angle, and the display properties of the FLCD are substantially independent of an angle of visibility.
As explained above, the FLCD is very promising as a large capacity OA display but inferior in display quality. Namely, the FLCD involves insufficient display gradations. Since the FLCD is basically bistable, it basically achieves binary display of black and white.
Conventionally, there are three methods that have been provided to increase the number of gray scales (gradations) of a ferroelectric liquid crystal display element. One technique is a so called domain size control method (for example, disclosed in Proceedings of the SID (Society for Information Display), Vol. 32/2, pp. 115 to 120, (1991) by W. J. A. M. Hartmann et al.), another technique is a so called pulse modulation method (for example, disclosed in National Technical Report Vol. 38, No. 3, pp. 313 to 317 (1992) by N. Wakita et al.), and still another technique is a so called dithering method (for example, disclosed in SID DIGEST (1991) pp. 261 to 264 by T. Yoshihara et al).
First, in the domain size control method, which may be called a texture-method, as described in Proceedings of the SID, Vol. 32/2, pp. 115 to 120, (1991) by W. J. A. M. Hartmann et al., a plurality of gradations can be obtained by controlling an inversion state of liquid crystal domains provided in one pixel. Namely, a molecular orientation of the liquid crystal provided in one pixel (element) is not uniform and is divided into some domains. The domain size control method controls the number of inversion of the divided domains, and changes the area of "Black" (or "White") in one pixel like a dithering method, so that a plurality of gradations can be obtained.
Next, in the pulse modulation method, as described in National Technical Report Vol. 38, No. 3, pp. 313 to 317 (1992) by N. Wakita et al., a plurality of gradations can be obtained by controlling the number of inversions of a drive voltage in a constant period by changing the pulse numbers. Namely, the pulse modulation method controls the pulse width of a pulse voltage to be applied to the liquid crystal element to increase the number of gray scales (gradations). Note, this pulse modulation method is broadly used in nematic liquid crystal device such as an STN-LCD, and the gradations can be largely increased by slowing the response time thereof.
Finally, in the dithering method, as described in SID DIGEST (1991) pp. 261 to 264 by T, Yoshihara et al., a plurality of gradations can be obtained by controlling the number of sub-pixels constituting one pixel. For example, one pixel is constituted by four or nine sub-pixels, and each of the sub-pixels is independently controlled as "White" or "Black". Note, this dithering method is well known and is also described in a part of Proceedings of the SID, Vol. 32/2, pp. 115 to 120, (1991) by W. J. A. M. Hartmann et al. Further, the dithering method is a technique similar to dot-photographs used in a newspaper, and the like.
The technique of changing the pulse width of a pulse voltage to be applied to liquid crystals does not sufficiently function with the present response speed of liquid crystals, so that it may display four gradations at the maximum. On the other hand, the dithering method requires a very large number of pixels, which increases the number of drive circuits and cost.
Note, the present invention method can also use the above method, as the present invention method and the prior art methods can be independently applied to a ferroelectric liquid crystal display element to increase the number of gray scales (gradations). Further, the present invention method can be applied not only to OA-equipment such as word processors and personal computers, but also applied to an electronic OHP display (with reference to SID DIGEST (1991) pp. 261 to 264 by T, Yoshihara et al.), and the like.